which two scenarios is this command required?

I think D may br a option here. Not sure though…

CD

A C are correct.

Why A:
http://docs.oracle.com/cd/E11882_01/backup.112/e10642/osadvsce.htm#BRADV90043
Begin recovery by executing the RECOVER command with the USING BACKUP CONTROLFILE clause. Specify UNTIL CANCEL if you are performing incomplete recovery. For example, enter:

SQL> RECOVER DATABASE USING BACKUP CONTROLFILE UNTIL CANCEL;

Apply the prompted archived logs. If you then receive another message saying that the required archived log is missing, then it probably means that a ******necessary redo record is located in the online redo logs. This situation can occur when unarchived changes were located in the online logs when the instance failed.*****

Sorry, NOT B & C
It is:
A & C:

A:
http://docs.oracle.com/cd/B19306_01/backup.102/b14191/recoscen.htm#BRADV229
“(…)
Recovering After the Loss of All Members of an Online Redo Log Group
(…)
If the group is current, then it is the log that the database is currently writing to and you should attempt to clear the log; if impossible, then you must restore a backup and perform incomplete recovery up to the most recent available redo log.”

C: https://asktom.oracle.com/pls/asktom/f?p=100:11:::::P11_QUESTION_ID:1143434607537

AC

for C: it’s evidently
for A: http://docs.oracle.com/database/121/BRADV/osadvsce.htm#BRADV90049

Table 31-4 Recovering After the Loss of an Online Redo Log Group
If the Group Is…
Current
And You Can…
Attempt to clear the log; if impossible, then you must either use Flashback Database or restore a backup and perform incomplete recovery up to the most recent available redo log.

A C

D- this point does not describe the problem
What we should restore (whole database, some datafiles ??) and why.
This point ONLY inform that after database backup – controlfile backup was taken (which is
good practise imho).

AC